1. Field of the Invention
This invention relates to a focusing arrangement for a video camera, and especially to an arrangement for aiding manual focusing through an electronic viewfinder. More specifically, the invention pertains to a signal processing circuit of the type that isolates a focus-related characteristic of a video signal and uses the isolated characteristic to visually modify the display in the viewfinder.
2. Description Relative to the Prior Art
In a manually focused video camera, an electronic viewfinder is not only used to compose a scene as recorded by the camera but also to determine when the scene image is properly focused. The proper focus, however, is not a discrete distance but a range of distances through which the image is acceptably defined, i.e., a range indicated by the depth of field for a particular optical (lens) aperture and subject distance. An electronic viewfinder repeats images of the scene at the television frame rate, i.e., 1/30 second. This "exposure interval" dictates a particular lens aperture for the existing light condition and, therefore, establishes a particular depth of field for each subject distance. A special problem emerges when the viewfinder is used with a video still camera, which provides several exposure interval and aperture combinations for each light condition. The light condition that dictates a certain aperture . . . and thus a certain depth of field . . . for the electronic viewfinder may require an entirely different aperture for the still exposure (because the exposure interval may not be 1/30 second). Thus the depth of field for viewing may be an artificial indication of the actual picture-taking condition. The "real" depth of field may be considerably compressed from that observed in the viewfinder with the attendant possibility that a properly-focused viewfinder display may be unfocused with respect to the recorded picture.
If one could reliably locate the central focus position for the displayed depth of field, the picture would be in focus for any aperture (that is, for any depth of field). Owing, however, to the small size of the display screen in the viewfinder and the limited bandwidth available for the display, differences between details shown on the viewfinder screen are not very sharp or clear. Searching for optimum focus, necessary in view of the depth of field considerations heretofore mentioned, is a procedure marked by uncertainty.
U.S. Pat. No. 4,660,092 suggests one arrangement for dealing with such problems and finding the central focus position. A high frequency component of a luminance signal generated by a video camera is converted into a d.c. control signal having an amplitude that varies with high frequency content. By using the control signal to adjust the gain applied to the luminance signal in a central area of the viewfinder display, the contrast of the picture in the central area is accentuated according to the degree of focus. In practice, however, a contrast change in the tiny viewfinder display is insufficient for focusing differences to be visually striking.